The present invention relates generally to a starting current limiting device for single-phase induction motors used in household electrical equipment.
A household electrical equipment, such as an air conditioner, a refrigerator, a washing machine, or an electrical fan, ordinarily utilizes a single-phase induction motors for the driving purposes since the electrical power readily available from a socket is usually a single-phase AC voltage with a voltage of 110 V and a frequency of 60 Hz.
The induction motor of these household electrical equipment must usually be turned on and turned off repeatedly. For description purpose, the term "working current" is hereinunder defined as the current flowing from a power source into an induction motor, the term "starting current" as the working current when an AC voltage is just applied to an induction motor, and the term "rating current" as the working current when the induction motor is running under a steady state. It is found that the starting current is usually about three to six times as large as the rating current. The extremely large starting current results from the initial high induced potential required to overcome the inertia of the rotor of the induction motor so as to accelerate the rotor.
FIG. 1 shows a characteristic curve of the working current when the starting current limiting device according to the present invention is not used. At an instant t=t.sub.1 the power switch is turned on, and the working current arises abruptly from zero to a starting current of I.sub.S and then gradually descends therefrom following a curve "a". The value of the I.sub.5 is about four times as large as the value of the rating current I.sub.R. At a time t=t.sub.2 the value of the working current reaches the rating value I.sub.R, and from this time on the working current keeps steady at the rating value I.sub.R, as represented by a curve "c". During this period, the induction motor runs under a steady state. At a time t=t.sub.3 the power switch is turned off and the current gradually falls to zero, as represented by a curve "d". At a time t=t.sub.4 the power switch is turned on again and identical characteristic curve of working current is repeated.
In the period between t=t.sub.2 and t=t.sub.3 the working current converts substantially all its electrical energy to kinetic energy of the rotor. However, in the initial period between t=t.sub.1 and t=t.sub.2, the rotational speed of the rotor, due to its inertia, is not able to "catch up with" the high starting current. Therefore, most of the electrical energy carried by the starting current at this time would be converted into heat energy dissipated in the coil, rather than into kinetic energy of the rotor of induction motor. A curve "b" in the figure represents the current induced by the rotation of the rotor. The amount of electrical energy dissipated in the coil is shown by the shaded area between curve "a" and curve "b".
Therefore, since the single-phase induction motors used in household electrical equipment are frequently turned on and turned off, the dissipated electrical energy will be accumulated to a considerable extent.
In addition, since the starting current is large, the acceleration force exerted upon the rotor is also large. The rotor thus suffers from a larger mechanical loss.
Further, the extremely large starting current abruptly applied to the induction motor will inevitably cause a significant voltage drop on other electrical appliances connected to the same power source, and thus adversely affect the operations of those appliances.